Hippolyte Dourdent | LIP6 - Équipe QI

Hippolyte Dourdent - Unambiguous non-locality without entanglement = Deterministic classical non-causality

Wed, 17 Dec 2025 00:00:00 +0000

Unambiguous non-locality without entanglement = Deterministic classical non-causality

Ce séminaire, donné par Hippolyte Dourdent, aura lieu le 17 December 2025, à 13:0. Il aura lieu en salle 25-26 105.

Vous trouverez un plan du campus ici.

Résumé

Process functions generalize deterministic classical communication by removing the assumption of a fixed causal structure between free local operations without generating paradoxes. Kunjwal and Baumeler [PRL 131, 120201 (2023)] showed that any Boolean local operations with a process function lacking a global past can be encoded in a multi-qubit product basis that cannot be projected onto using local operations with classical communication—an instance of quantum nonlocality without entanglement (QNLWE). We extend this result to arbitrary dimensions and any number of parties by linking the unique fixed-point condition defining process functions to a simple unambiguity condition requiring locally disjoint operations. After refining previous characterisations of process functions and characterising the notion of (non-)causal process functions, we show that: (i) every unambiguous product basis yields a process function; (ii) every process function admits an encoding in an unambiguous product basis; and (iii) non-causal process functions corresponds to unambiguous QNLWE bases. This establishes a systematic framework for constructing and analyzing these objects. Notably, it implies that certain causal inequalities maximally violated by a process function correspond directly to non-signaling inequalities. Furthermore, it provides a new interpretative perspective on paradox-free deterministic classical communication, formulated in terms of event labeling.

Network-Device-Independent Certification of Causal Nonseparability

Wed, 30 Oct 2024 00:00:00 +0000

Causal nonseparability is the property underlying quantum processes incompatible with a definite causal order. So far it has remained a central open question as to whether any process with a clear physical realisation can violate a causal inequality, so that its causal nonseparability can be certified in a device-independent way, as originally conceived. Here we present a method solely based on the observed correlations, which certifies the causal nonseparability of all the processes that can induce a causally nonseparable distributed measurement in a scenario with trusted quantum input states, as defined in [Dourdent et al., Phys. Rev. Lett. 129, 090402 (2022)]. This notably includes the celebrated quantum switch. This device-independent certification is achieved by introducing a network of untrusted operations, allowing one to self-test the quantum inputs on which the effective distributed measurement induced by the process is performed.

Semi-device-independent Certification of Causal Nonseparability with Trusted Quantum Inputs

Fri, 26 Aug 2022 00:00:00 +0000

While the standard formulation of quantum theory assumes a fixed background causal structure, one can relax this assumption within the so-called process matrix framework. Remarkably, some processes, termed causally nonseparable, are incompatible with a definite causal order. We explore a form of certification of causal nonseparability in a semi-device-independent scenario where the involved parties receive trusted quantum inputs, but whose operations are otherwise uncharacterised. Defining the notion of causally nonseparable distributed measurements, we show that certain causally nonseparable processes which cannot violate any causal inequality, such as the canonical example of the quantum switch, can generate noncausal correlations in such a scenario. Moreover, by further imposing some natural structure to the untrusted operations, we show that all bipartite causally nonseparable process matrices can be certified with trusted quantum inputs.